In apparatus for the handling of continuous lengths of flexible films, such as packaging equipment which advances film from a spool to its point of use, cylindrical rolls are utilized to advance the film without wrinkling or other distortions in the film being handled. Such roll mechanisms usually consist of a drive or power roll, and a driven or passive roll which rests upon the drive roll and is rotated either by frictional engagement with the drive roll or by the enmeshing of gears located on the ends of the roll axles. The film passes through the line of contact between the two rolls, known as the "nip", and is thereby held firmly for uniform propulsion toward its point of use.
To ensure that the film is uniformly gripped in its passage through the nip of the rolls, the surfaces of the rolls must be perfectly mated at the nip line. This requires not only a high degree of perfection in the cylindrical surfaces of the rolls, but also requires precise juxtaposition of the rolls so that the nip line is unbroken and perfectly perpendicular to the direction of film travel. The difficulties in achieving a perfect nip line are particularly accentuated when the films are very thin (e.g. four mils or less) and have widths greater than five inches.
In most film advancing equipment, it is preferable to utilize passive rolls which can be quickly and easily removed from engagement with the drive roll. The purpose of this is to facilitate string-up or threading of the film through the equipment, and to simplify clean-out of the equipment in the event of malfunctions where film may pile up or jam in the rolls. Such desirable feature of adjustability of the passive roll for quick engagement or disengagement with the drive roll further accentuates the difficulties in achieving a precision nip, particularly after repeated cycles of engagement and disengagement.
Disengageable passive roll assemblies have generally been equipped with spring or counterweight mechanisms to place the roll under a tension forcing it into contact with the drive roll. In the course of long term use, however, spring tension diminishes in an unpredictable manner, causing unreliable performance. Another problem encountered with spring-urges and counterweighted rolls is that a chatter may occur wherein the resilient nature of the restoring force permits a slight bouncing of the roll over surface blemishes in either the rolls or the film.